1. Field of the Invention
The present invention generally relates to an electrochromic display apparatus and a method for manufacturing the same.
2. Description of the Related Art
Recently, the amount of development of the electronic paper has been increasing. The electronic paper has been watched as an electronic medium which replaces paper. Since the electronic paper can be treated like the paper, features that are different from those of a CRT or a liquid crystal display are required. For example, features such that the electronic paper is a reflective display, and has high white reflectivity and a high contrast ratio are required. Further, for example, features that the electronic paper is a high definition display, and has a memory effect of display are required. Furthermore, for example, features that the electronic paper can operate at a low voltage, and is thin, light-weight and inexpensive are required. Among these features, there is a high requirement particularly for the white reflectivity and the contrast ratio that are equivalent to those of the paper.
An electronic paper which utilizes, for example, a reflection-type liquid crystal, an electrocataphoresis or a toner migration has been proposed. However, it is difficult to display multicolor while achieving high white reflectivity and high contrast ratio by any type of the electronic paper described above. In general, in order to display multicolor, a color filter is attached to a display apparatus such as the electronic paper. Since the color filter absorbs light, a reflection ratio of the display apparatus is decreased in a case where the color filter is attached to the display apparatus. Since each pixel of the color filter is divided into three parts of red (R), green (G) and blue (B), the reflection ratio of the display apparatus is decreased and thereby the contrast ratio is decreased. In a case where the white reflectivity and the contrast are decreased greatly, visibility of the display apparatus decreases greatly. Thus, it becomes difficult to use the display apparatus as the electronic paper.
On the other hand, there is a promising technique for realizing the reflective display in that an electrochromic phenomenon is utilized and the color filter is not utilized. According to the electrochromic phenomenon, a color which is displayed from an electrochromic compound is changed in a reversible fashion by a reversible oxidation-reduction reaction when a voltage is applied to the electrochromic compound. The electrochromic display apparatus utilizes color change of the reversible oxidation-reduction reaction of the electrochromic compound. The color change is performed by controlling display and nondisplay of the color of the electrochromic compound. Since the electrochromic display apparatus is a reflective display, has a memory effect of display, and can operate at a low voltage, the electrochromic display apparatus has been widely developed as a promising candidate for the electronic paper in a wide variety of fields from material development field to device design field.
However, the electrochromic display apparatus has a disadvantage in that a response speed of changing the display/nondisplay state of color is relatively slow, since the electrochromic display apparatus utilizes the reversible oxidation-reduction reaction. Patent document 1 (for example, Japanese Patent Laid-Open Publication No. 2001-510590) discloses a technique in which the response speed is improved by disposing the electrochromic compound closer to the electrode. According to patent document 1, the response speed is improved from several tens seconds to almost 1 second in the cases where the electrochromic display apparatus displays blue, and the electrochromic display apparatus erases blue. However, the improved response speed is not enough; thus it is necessary to improve still further the response speed in developing the electrochromic display apparatus. Since the electrochromic display apparatus can display various colors by designing compositions of the electrochromic compounds, the electrochromic display apparatus is expected to be used as a multicolor display apparatus.
There are some examples of the multicolor display apparatus which utilize the electrochromic phenomenon. For example, patent document 2 (for example, Japanese Patent Laid-Open Publication No. 2003-121883) discloses a multicolor display apparatus which utilizes electrochromic compound layers that are formed by stacking plural kinds of electrochromic compounds. According to patent document 2, each of the plural electrochromic compounds is included in a functional group. The functional groups are different from each other, and have different voltages that are necessary for displaying colors. Each of the functional groups displays a different color.
Patent document 3 (for example, Japanese Patent Laid-Open Publication No. 2006-106669) discloses a multicolor display apparatus which includes multi-electrochromic layers formed on an electrode. The multicolor display apparatus according to patent document 3 displays multicolor (multiple colors) by utilizing voltage difference or current difference of the multi-electrochromic layers that are necessary for displaying colors. The multi-electrochromic layers are formed by stacking or mixing plural electrochromic compounds. Each layer of the multi-electrochromic layers displays a different color, and voltages and electric charges that are necessary for displaying colors are different from each other.
Patent document 4 (for example, Japanese Patent Laid-Open Publication No. 2003-270671) discloses a multicolor display apparatus which includes plural units of plural pairs of transparent electrodes and plural electrochromic layers. In each unit, the electrochromic layer is held between the pair of the transparent electrodes. The plural units are stacked on each other. Patent document 5 (for example, Japanese Patent Laid-Open Publication No. 2004-151265) discloses a multicolor display apparatus which includes the units of patent document 4 and a passive matrix panel or an active matrix panel. The multicolor display apparatus according to patent document 5 displays three colors corresponding to RGB.
Herein, patent document 6 (for example, Japanese Patent Laid-Open Publication No. 2009-163005) discloses a multicolor electrochromic display apparatus that solves problems described above with regard to patent documents 2 to 5. The multicolor electrochromic display apparatus includes plural display electrodes that are disposed between a display substrate and an opposed electrode and are separated from each other. The multicolor electrochromic display apparatus includes plural electrochromic layers that are formed on the plural display electrodes, respectively.
However, the multicolor electrochromic display apparatus includes problems as described below.
The electrochromic display apparatus was introduced as an electrochemical element which utilizes the Grätzel cell that was introduced in 1991. The electrochromic display apparatus includes a nanoporous particle layer which has a large surface area, and compounds that cause electrochromic reaction. The compounds are attached to or absorbed in the nanoporous particle layer. In a case where the compounds are poorly attached to or absorbed in the nanoporous particle layer, unevenness of compound concentration may occur. In order to cause a sufficient electrochromic reaction, it is necessary for electrolytes of the compounds to penetrate into the nanoporous particle layer. Since the electrochromic display apparatus includes the nanoporous particle layer, gas bubbles may remain in the nanoporous particle layer.
Accordingly, these matters as described above may cause unevenness of display, unevenness of response speed or delay of response of the electrochromic display apparatus. The larger a display area of the electrochromic display apparatus becomes, the more likely those problems are to occur. In a case where plural nanoporous particle layers are stacked, the electrolytes may not penetrate into the plural nanoporous particle layers immediately. Thus, the problems as described above may occur remarkably.
In accordance with a single element of a multifunction electrochemical element in which plural electrochemical function layers are stacked, the plural electrochemical function layers must be sufficiently filled with an electrolyte medium, and ion migration must be performed sufficiently, in order to cause each layer of the plural electrochemical function layers to function.